drm/display: Fix building with GCC 15

GCC 15 enables -Werror=unterminated-string-initialization by default.
This results in the following build error

drivers/gpu/drm/display/drm_dp_dual_mode_helper.c: In function ‘is_hdmi_adaptor’:
drivers/gpu/drm/display/drm_dp_dual_mode_helper.c:164:17: error: initializer-string for array of
 ‘char’ is too long [-Werror=unterminated-string-initialization]
  164 |                 "DP-HDMI ADAPTOR\x04";
      |                 ^~~~~~~~~~~~~~~~~~~~~

After discussion with Ville, the fix was to increase the size of
dp_dual_mode_hdmi_id array by one, so that it can accommodate the NULL
line character. This should let us build the kernel with GCC 15.

Signed-off-by: Brahmajit Das <[email protected]>
Reviewed-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Jani Nikula <[email protected]>

Author: Brahmajit Das

drivers/gpu/drm/display/drm_dp_dual_mode_helper.c

@@ -160,11 +160,11 @@ EXPORT_SYMBOL(drm_dp_dual_mode_write);
 
 static bool is_hdmi_adaptor(const char hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN])
 {
-	static const char dp_dual_mode_hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN] =
+	static const char dp_dual_mode_hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN + 1] =
 		"DP-HDMI ADAPTOR\x04";
 
 	return memcmp(hdmi_id, dp_dual_mode_hdmi_id,
-		      sizeof(dp_dual_mode_hdmi_id)) == 0;
+		      DP_DUAL_MODE_HDMI_ID_LEN) == 0;
 }
 
 static bool is_type1_adaptor(uint8_t adaptor_id)